Hydrocarbon surfactants are very common and are the workhorse materials for making aqueous foams and wetting surfaces, among other things. Surfactants, in general, produce these effects by lowering the surface tension of the liquid in which they are dissolved. Hydrocarbon surfactants, however, lower the surface tension of water to only about 30 dynes/cm (0.03 N/m). To provide lower surface tensions, fluorosurfactants are often used.
Fluorosurfactants, including amphiphilic fluoropolymers, can give surface tensions of less than 20 dynes/cm (0.02 N/m), which are necessary to better wet low energy surfaces, for example. Like hydrocarbon surfactants, fluorosurfactants can be classified as anionic (containing negative charge), cationic (containing positive charge)), amphoteric (having both positive and negative charges) and nonionic (having no charge). Nonionic fluorosurfactants are particularly desirable due, not only to the very low surface tension obtainable, but also to their efficacy in extreme conditions such as high acidity or alkalinity, high salt levels (ionic strength), elevated temperatures, etc. They are also very resilient and not easily degraded.
One type of nonionic fluorosurfactant is an alkoxylated fluoroalcohol. The fluoroalcohol molecule itself is hydrophobic so that it is insoluble in water or other aqueous fluids. Alkoxylation of the fluoroalcohol adds a hydrophilic portion to the molecule so that it is water soluble and surface active.
Alkylene oxides, such as ethylene oxide and propylene oxide, are reactive toward most hydroxyl functional groups such as the terminal —OH of the fluoroalcohol or alkoxylated fluoroalcohol. In many cases, however, catalysts are employed to aid in the alkoxylation reaction. Existing catalysts for alkoxylating fluoroalcohols include boron trifluoride (BF3 or BF3-etherate), borohydrides, etc. These materials have certain shortcomings, however. Boron trifluoride is both toxic and corrosive and presents a safety and handling concern. Furthermore, it has a fairly low reactivity, broad alkoxylate distributions and requires excessive residual starting materials. Boron trifluoride also produces undesirable by-products, namely, HF and 1,4-dioxane. Borohydride catalysts for alkoxylation of fluoroalcohols are effective, but only with the addition of other additives, and are sensitive to impurities. Without the proper balance of additives and low impurities, borohydride catalysis for alkoxylating fluoroalcohols can be slow or fail outright. Borohydrides also present a handling and safety concern, because the borohydride powders are highly flammable and produce highly flammable hydrogen gas as a reaction byproduct.
There is therefore a need to provide a method of alkoxylating fluoroalcohols that overcomes these disadvantages of borohydride and boron trifluoride catalyst materials.